Billionaire investor George Soros is defending his right to donate hundreds of thousands of pounds to a pro-European Union campaign aimed at stopping the U.K. leaving the political and economic bloc.

The tycoon had been heavily criticized after it was revealed that, through one of his foundations, he had donated a sizable amount of funds to the Best for Britain campaign, which advocates stopping Brexit and keeping the U.K. in the EU.

In response, Soros hit back at what he described as "toxic, personal criticism" and dismissed accusations that his actions were "undermining democracy."

Writing in the Mail on Sunday newspaper, he said: "I consider Brexit a tragic mistake. … Brexit is a lose-lose proposition both for Britain and for Europe. Politically, Europe without Britain will be weakened in its ability to defend and promote democratic values."